In the art of chemical pulping of comminuted cellulosic fibrous material, for example, wood chips, the cellulose material is typically treated with cooking chemicals under pressure and temperature in one or more cylindrical vessels, known as digesters. This treatment can be performed continuously or in a batch mode. In the continuous mode, chips are continuously fed into one end of a continuous digester, treated, and essentially continuously discharged from the other end. In the batch method, one or more batch digesters are filled with chips and cooking chemical, capped and then treatment commences. Once the treatment is finished the contents of the batch digester are discharged. In either batch or continuous (e.g. Kamyr.RTM.) digesters, discharge of the slurry of essentially fully-cooked comminuted cellulosic fibrous material, that is, wood pulp, is aided by introducing diluting liquor to the vicinity of the discharge. This dilution liquor not only aids in the discharge of the pulp but also cools the pulp and acts to control the liquid content, that is, the consistency, of the discharged slurry.
This process of diluting the pulp prior to discharge has been practiced since the introduction of the continuous digester in the late 1930s and early 1940s. For example, U.S. Pat. No. 2,474,862, having a priority date of Oct. 15, 1942, clearly illustrates the addition of cooling liquid to the outlet of one of Johan Richter's earliest continuous digesters. Similar dilution flows to the outlet of the earlier continuous digesters are shown in Richter's "The History of Kamyr Continuous Cooking" (1981) and in U.S. Pat. No. 2,938,824. In the late 1950s, it was recognized that introducing cooling dilution to the outlet of a continuous digester to cool the pulp before discharging the pulp under pressure, that is, "blowing", produced a stronger pulp. This concept became known as "Cold Blowing".
In the early 1960s, the concept of counter-current treatment of cellulose material in a continuous digester was first introduced, for example, as described in U.S. Pat. Nos. 3,097,987 of Sloman and 3,007,839 of Richter. Subsequently, counter-current washing in the bottom of the digester was introduced, as described in U.S. Pat. Nos. 3,200,032 of Richter, et al. and 3,298,899 of Laakso. This washing process was marketed under the trademark HI-HEAT. Similar combinations of counter-current HI-HEAT washing and cooling Cold Blow dilution are described in U.S. Pat. Nos. 3,380,833; 3,413,189; 3,425,898; 3,429,733; 3,445,328; 3,427,218; 3,532,594; 3,579,418; and 3,811,994. However, in all of the systems illustrated in these patents the cooling dilution liquor is introduced by a series of nozzles distributed uniformly around the periphery of the digester, generally directed perpendicular to the vessel shell dimension of elongation and typically all communicating with a common ring header. For example, 3,007,839 illustrates dilution nozzles directed inward and then downward toward the discharge agitator, or "outlet device".
Even more recent patents, for example, 5,236,553; 5,470,437; 5,567,280; and 5,620,562 all disclose digesters in which the dilution liquor above the outlet is introduced using radial nozzles.
However, the recent developments in the area of material handling, in particular material handling in chip bins and continuous digesters, indicates that such localized introduction of dilution to comminuted cellulosic fibrous material in vessels interferes with the uniform movement of the material in those vessels. Recent U.S. Pat. Nos. 5,500,083; 5,617,975; and 5,628,873 disclose novel methods of handling and treating wood chips so that uniform movement and treatment is ensured. Copending application 08/936,047 filed on Sep. 23, 1997 now U.S. Pat. No. 5,985,096 discloses novel methods to apply these principles to the design of digester vessels. These Patents and applications disclose that movement of a slurry of comminuted cellulosic fibrous material and liquid can be dramatically affected by the uniformity of the introduction of liquids to the digester. Non-uniform dilution, for example, can result in localized variation in slurry consistency and produce localized variation in the flow patterns along the vessel internal shell diameter. In the vertical flow of material, any variation in flow characteristics at one level, for example, the friction between the material and the vessel wall, can affect the character of the flow above it. This is particularly the case where dilution is introduced at the bottom of a digester. Typically, this is the only location where liquids are introduced radially inward from the vessel shell. Other liquor introductions, for example, by means of cooking circulations, are introduced by means of a centrally-located conduits, that is, via a "center pipe", which have little direct influence upon the character of the flow along the internal diameter of the vessel. Thus, the radial, localized, non-uniform introduction of liquor at the bottom of the digester can have a significant effect upon the flow of material in the entire digester.
The present invention provides a method and apparatus for introducing liquids to a digester vessel that overcomes the non-uniformities in treatment and material movement that characterizes the prior art. One embodiment of this invention comprises a method for distribution of liquid to a digester or other vessel having a flow path for comminuted cellulosic fibrous material having at least one radial or step increase from a first diameter to a second larger diameter, comprising or consisting of the steps of (a) causing the material to flow past the step increase so that an annular void is created between the material and the second diameter; and (b) introducing liquid into the annular void so that the liquid is distributed around the circumference of the flow path. In one embodiment of this invention, liquid may be introduced to the void by way of one or more nozles which communicate directly with the void. A preferred device for introducing the liquid is a screen plate located in the vicinity of the step increase. This screen plate may be a right cylindrical screen having an internal diameter substantially equal to the second diameter. The screen may also be a right conical screen plate having a first diameter substantially equal to the first diameter of the flow path and a second diameter essentially equal to the second diameter of the flow path.
The method of introducing liquids to a digester or other vessel is preferably practiced in the lower part of the digester or other vessel, but may be used anywhere along the height of the digester or other vessel where liquor introduction is desirable, and may be provided at more than one position along the vessel.
Particularly, according to the invention a method of treating or producing chemical pulp (such as kraft pulp) using a vessel (typically a vertical vessel) having an interior surface, is provided. The method comprises the steps of: (a) Causing a cellulose slurry to flow in the vessel interior in a flow path. (b) Forming a substantially annular void at at least one point along the flow path. And, (c) introducing dilution or treatment liquor into the substantially annular void so that the liquor is substantially uniformly distributed about the periphery of or into the slurry.
Step (b) may be practiced by providing a horizontal step-out in a vertical vessel. Step (c) may be practiced by causing treatment or dilution liquid to flow into the substantially annular void at a plurality of openings substantially evenly spaced around the substantially annular void. Step (a) is typically practiced causing the slurry to flow substantially downwardly in the vessel. Steps (b) and (c) may be further practiced by disposing a screen adjacent the substantially annular void, the liquid being introduced through the screen. Steps (b) and (c) may also be practiced by providing the screen surface substantially parallel to the flow path, or by providing a substantially conical screen surface which makes an angle of about 30-60.degree. with respect to the flow path.
Step (c) may be practiced by introducing treatment or dilution liquid into a first annular cavity within the vessel, causing the liquid to flow through a plurality of orifices into a second annular chamber within the vessel interior, and then through a screen surface into the vessel interior. Step (c) may be practiced by passing the liquor through a screen surface adjacent the annular void.
The invention also relates to a chemical pulp producing or treating assembly comprising the following components: A substantially vertically elongated vessel, having an interior. Means for forming a substantially annular void volume at at least one location along the vessel interior. And, means for introducing a treatment or dilution liquid into the void.
The introducing means may comprise one or more nozzles which directly communicate with the void, but preferably include a screen surface adjacent the void inside the vessel interior and the annular void forming means may comprise a step out. The introducing means may further comprise a first header inside the vessel interior having a substantially annular chamber in communication with the screen surface, and the introducing means may comprise a plurality of openings in addition to the screen surface. The introducing means may further comprise a second header immediately adjacent the screen surface, and further comprising a plurality of orifices between the first header and the second header allowing liquid flow therebetween. The screen surface may be either substantially vertical, or substantially conical (e.g. at an angle of 30-60.degree. to the vertical).
According to another aspect of the present invention a chemical pulp producing or treating assembly is provided comprising: A substantially vertically elongated vessel having an interior. An extraction or recirculating screen assembly having a first screen surface diameter. A step out substantially immediately below the extraction or recirculating screen assembly. A liquid introducing screen assembly substantially immediately below the step out, and having a second screen surface diameter greater than the first screen diameter by at least one percent (e.g. 2-5%, and typically about 1-12 inches preferably about 1-3 inches). A substantially annular header communicating with the liquid introducing screen assembly. And, liquid being fed to the liquid introducing screen assembly for substantially uniform introduction into pulp moving downwardly in a slurry in the vessel.
The second screen surface may be substantially vertical, or conical. The invention may further comprise a plurality of conduits introducing liquid into the header, and a plate positioned in front of each conduit to deflect the flow of liquid from the conduit so that the liquid does not flow immediately into contact with the second screen surface. The header may comprise a first header, and the assembly may further comprise a second header immediately adjacent the first header, with a plurality of orifices between the first and second headers.
It is the primary object of the present invention to provide for the uniform introduction of dilution or treatment liquid into chemical pulp in a vessel. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.